Sialon containing high content of alpha prime phase

ABSTRACT

Silicon aluminum oxynitride materials having high hardness and high density are formed by pressureless sintering of silicon nitride, yttrium oxide and aluminum nitride. The materials contain at least 75 weight percent of alpha prime phase sialon.

This application is a continuation of application Ser. No. 07/127,846,filed Dec. 2, 1987, now U.S. Pat. No. 4,873,210.

This invention concerns silicon aluminum oxynitride materials, commonlyreferred to as sialons. Such materials are disclosed in U.S. Pat. No.4,127,416. This invention is particularly concerned with articles ofsuch materials formed by pressureless sintering that have high hardness,high density and contain at least about 75% of the alpha prime phasesialon. U.S. Pat. No. 4,563,433 discloses double phase sialonscontaining 20 to 90 weight percent of beta phase sialon and 10 to 70weight percent of alpha phase sialon. Z. Huang et al, "Formation ofAlpha Si₃ N₄ Solid Solutions in the System Si₃ N₄ -AlN-Y₂ O₃."Communications of the American Ceramic Society, June, 1983, pp. C96-C97,discloses compositions containing alpha sialon which were formed by hotpressing.

Sialon articles are being used in various industrial applications, suchas cutting tools, blades and wear-resistant parts, as described in U.S.Pat. No. 4,127,416. These articles comprise materials which arepredominantly single phase beta sialons, although they may contain up to20% of an intergranular phase.

Sialons as per the instant invention have higher hardness and higherwear resistance than presently available commercial materials disclosedin U.S. Pat. Nos. 4,127,416 and 4,563,433. Sialons as per this inventioncomprise at least about 75% of the alpha prime phase sialon and arelocated in a small portion of the quasiternary diagram for siliconnitride, aluminum nitride and yttrium oxide. The sialons may alsocontain a small amount of an intergranular phase which is not detectableby routine x-ray diffraction examination.

In the drawings,

FIG. 1 is a diagram of the quasiternary system Si₃ N₄ -AlN-Y₂ O₃. FIG. 1also shows triangle XYZ which is a small portion of the diagram.

FIG. 2 is an expanded version of triangle XYZ of FIG. 1 and shows thelocation of pressureless sintered sialon compositions in accordance withthis invention.

The raw materials used in making sialon articles as per this inventionwere silicon nitride powder, grade E-10, manufactured by Ube Industries,Ltd., aluminum nitride powder, grade AP-10, manufactured by Denki KagakuK. K., and yttrium oxide powder, grade 5600, manufactured by Molycorp,Inc.

The process used was to dry ball mill mixtures of the three powdersusing silicon nitride grinding media. For 100 gram mixtures, 2,000 gramsof grinding media were used and milling time was 20 hours. For 400 grammixtures, 16,000 grams of grinding media were used, and milling time was36 hours. The milled powder mixtures were then pressed at 20,000 psiinto articles such as disks or bars which were then fired in siliconnitride crucibles with their own bedding powder. The samples were firedabove 1700° C., typically between 1740° and 1800°. Most studies wereconcentrated on firing at about 1760° to 1780° C. Samples were held atthe sintering temperature for about four hours. The pressurelesssintering was done in nitrogen flowing gas at one atmosphere pressure.

The fired materials were evaluated by standard x-ray diffractometer todetermine phases present. The x-ray diffraction patterns were recordedby use of a Ni filtered CuK alpha radiation at 45 KV, 10 mA, 1° slit.

EXAMPLE 1

This composition consisted of yttrium oxide, aluminum nitride andsilicon nitride at a molar ratio of 1:10:21. X-ray diffraction analysisindicated that this material consisted of about 80 weight percent of thealpha prime phase sialon and about 20 weight percent of beta siliconnitride. This material had a hardness (R45N) of 91.3 psi, a sintereddensity of 3.247 and a strength (3 pt. bend) of 82,000 psi.

EXAMPLE 2

This composition consisted of yttrium oxide, aluminum nitride andsilicon nitride at a molar ration of 1:11:21. The sintered material wasabout 100% of the alpha prime phase sialon (within X-ray diffractionexperimental limits) and had a hardness (R45N) of 91.2 psi, a sintereddensity of 3.244 and a strength (3 pt. bend) of 80,000 psi.

Table I shows the compositions, in weight percent, of examples withinthe scope of this invention.

                  TABLE I                                                         ______________________________________                                        Example   Y.sub.2 O.sub.3                                                                             AlN    Si.sub.3 N.sub.4                               ______________________________________                                        1         6.31          11.44  82.25                                          2         6.23          12.45  81.32                                          3         5.65          11.72  82.63                                          4         6.27          11.95  81.78                                          5         6.62          12.72  80.66                                          6         7.0           13.0   80.0                                           7         6.13          12.72  81.15                                          8         5.93          12.58  81.49                                          ______________________________________                                    

Examples 1 and 3, which are within Region A in FIG. 2, contained atleast about 75% of the alpha prime phase sialon, the balance (excludingthe previously mentioned small amount of intergranular phase) being betasilicon nitride.

Examples 2 and 4-8, which are within Region B in FIG. 2, were about 100%alpha prime phase sialon (again excluding intergranular), the amount ofbeta phase that is present and determinable by x-ray diffraction beingnone or negligible.

Table II shows the x-ray diffraction data, in 2θ, for alpha prime phasesialon, beta silicon nitride, Example 1 and Example 2. In Table II, VSmeans very strong, S strong, MS medium to strong, M medium, MW medium toweak, W weak, VW very weak.

                  TABLE II                                                        ______________________________________                                        Alpha          Beta                                                           Prime          Si.sub.3 N.sub.4                                                                         Example 1                                                                              Example 2                                  ______________________________________                                        13.10  (W)     13.35  (MW)                                                    20.4   (S)                  20.25                                                                              (M)   20.4 (M)                               22.77  (M)     23.25  (MW)             22.6 (W)                               26.35  (MW)    26.9   (S)              26.4 (W)                               30.75                                  30.62                                                                              (S)                                              33.6   (VS)  33.4 (W)                                          34.20  (S)                  34.0 (S)   34.05                                                                              (S)                               35.1   (S)                  35.0 (S)   34.95                                                                              (S)                                                           35.9 (MW)                                                        36.0   (VS)                                                    38.6   (MS)    38.9   (W)   38.6 (M)   38.5 (M)                               39.2   (VW)                            39.1 (W)                               39.9   (W)                             39.8 (W)                               41.5   (W)     41.4   (M)   41.45                                                                              (W)   41.35                                                                              (W)                               43.17  (MS)                 43.2 (MW)  43.1 (MW)                              46.5   (W)                             46.4 (W)                               47.7   (MW)    47.75  (W)              47.7 (W)                               48.5   (MW)    48.05  (W)              48.8 (W)                               51.3           49.9   (MW)             51.2 (W)                               55.6   (VW)    52.1   (M)                                                     57.23  (MW)    57.8   (MW)  57.35                                                                              (MW)  57.15                                                                              (W)                               59.0   (VW)    59.65  (W)                                                     59.6   (VW)                                                                   60.85  (W)     61.25  (M)                                                     62.0   (MW)                 62.05                                                                              (W)   61.95                                                                              (MW)                                             63.9   (M)                                                     64.05  (MW)    64.8   (MW)                                                    64.2   (MW)                 64.2 (W)   64.15                                                                              (MW)                              65.3   (MW)                 65.65                                                                              (W)   65.25                                                                              (W)                               65.65  (W)                                                                    68.90  (MW)    69.1   (VW)  69.05                                                                              (W)   68.9 (MW)                              70.05  (W)     70.1   (VS)                                                    72.0   (W)     70.8   (MW)                                                                   71.6   (MW)                                                    ______________________________________                                    

In FIG. 2, as material composition moves from left to right, that is,from Region A to Region B, the beta silicon nitride content diminishes.At Y₂ O₃ concentrations lower than those of Regions A and B, thecompositions did not pressureless sinter to a high density at 1780° C.Compositions to the right of Region B could be pressureless sintered toa good density, but they were very susceptible to pores.

I claim:
 1. A pressureless sintered silicon aluminum oxynitride articlehaving high density and high hardness and having a composition whichcontains about 100 weight percent of alpha prime phase sialon and islocated in Region B of the quasiternary system Si₃ N₄ -AlN-Y₂ O₃ shownin FIG. 2.